Juvenile offenders are disproportionately minorities and affected by HIV, sexually transmitted infections (STI), mental illness, and substance use.1-19 Evidence-based programs (EBP) are sorely needed in juvenile justice settings, yet few exist, and for those that do, implementation has been slow, challenging, and rarely sustained. Yet without intervention, young offenders continue to engage in risk behaviors while on probation, amplifying their own and worsening already serious health disparities. Altering this trajectory is public health priority. This study builds on a 10-year collaboration between the investigative team and Cook County Juvenile Probation and Court Services (the second largest juvenile probation system in the U.S.) to develop and deliver PHAT Life, a uniquely-tailored comprehensive risk reduction EBP for young offenders. Findings from PHAT Life's recent efficacy trial reveal sustained reductions in sexual risk taking and substance use at 12-month follow up. The next step is to identify an implementation strategy that is both sustainable and cost-effective within juvenile probation. Few previous studies have compared peer- vs. adult- led behavior change programs and existing studies suffer from significant methodological deficiencies.20 This study will compare the effectiveness, cost-effectiveness, and sustainability of two implementation strategies, peer-led vs. probation staff-led PHAT Life, in juvenile probation's Evening Reporting Centers. Peers will be young adults formerly involved in juvenile justice who serve on the Juvenile Advisory Council (Youth Representatives). Both Youth Representatives and probation staff will receive extensive training, supervision, and fidelity monitoring. We will compare the impact, costs, and cost-effectiveness of PHAT Life on 300 13-17 year-old male and female offenders' risky sex, STI, substance use, and theoretical mediators when delivered by Youth Representatives vs. probation staff. Using qualitative methods, we will elicit the views of Youth Representatives, probation staff, and probation administrators regarding strategies to enhance PHAT Life's sustainability within juvenile justice, and we will explore the impact delivering PHAT Life on Youth Representatives' sexual risk, substance use, and other important outcomes. This proposal answers a compelling need for EBP in juvenile justice settings by addressing the interrelated issues of HIV/STI risk, substance use, and mental illness among probation youth, and their negative long-term trajectories. Comparing two implementation strategies--peers vs. probation staff--to determine the more effective and cost-effective approach will expedite uptake, delivery, and sustainability of much-needed EBP for teens who are at greatly elevated risk for HIV and other STIs.